The present invention relates to high speed automated machines for performing machine operations on a workpiece, and more particularly to tool changer systems for such automated machines.
A preferred application to which the invention relates is that of printed circuit board drilling. Printed circuit boards are in universal use today to mount and interconnect electrical components forming electrical circuitry. Typically the leads of the components are inserted through holes drilled in the boards to form predetermined hole patterns. Various systems are known in the art for automated drilling of the holes in printed circuit boards. One such system is described in U.S. Pat. No. 4,761,876. The system described therein is particularly well suited to drilling very small diameter holes, and describes a tool changer which employs a plurality of tool magazines and clips to hold the tools with the tools being secured or released by a top plate which captures the tools in a shank groove of reduced c ross-sectional dimension. The groove is formed in either the shank or in a tool collar which is fitted on the shank.
The tool changer system of the '876 patent works well. the use of tools with the groove formed directly in the shank works particularly well, since for high speed operation, the tool is well balanced. The use of a collar can lead to balance problems at very high speeds of operation, unless care is taken in the fabrication and fitting of the collar to the drilling tool. However, a disadvantage of the tool changer system of the '876 patent is the additional cost of either forming the groove in the shank or providing the collar. For example the cost of machining the groove on the shank of a drilling tool can add as much as $.50 to the cost of each tool, while a plastic collar can add $.10 to cost of the tool. A given drilling system with multiple spindles such as the system described in the '876 patent which undergoes heavy use can use hundreds of thousands of drilling tools in a single year of operation. The added cost of machining the grooves in the tool shanks or fabricating and fitting the plastic collars to the tool can add tens of thousands of dollars to the operating cost of a single multiple spindle drilling system. This is so even if the drills are sharpened and reused several times.
The use of sharpened tools leads to another problem. Whereas new tools have a given or known nominal length, tools lose some of their length as a result of being sharpened. It is desirable to have a known length of tool extending from the drilling spindle during the drilling operation, so that the length of the drilling stroke does not carry the tool tip too far, or fail to extend the drill tip completely through the workpiece or as far as desired. In the past, some systems have relied on the placement of the collar to register the insertion position of the drill in the spindle. The use of such collars leads to cost and balance disadvantages as described above.
It is therefore an object of the present invention to provide a method and apparatus which can reliably change tools used in an automated machine tool without requiring the tools to be modified, and which can insert tools of various lengths in the working unit with a given length of tool extending therefrom.